1. Field of the Invention
The present invention is directed to constructions for expansion joints for ducting, for example, such as may be used for the exhaust for large stationary gas turbines for the generation of electrical power, and for other ducting environments, especially those involving high temperature.
The present invention is directed, in particular, to seals for such expansion joints and, more specifically, to a molded polytetrafluoroethylene (PTFE, e.g., sold under the mark Teflon(copyright)) expansion joint for providing continuous contact with a coated fiberglass belt.
2. Prior Art
Ducting which is used to transport air or other gaseous flows that are subject to high temperatures, typically must be provided with expansion joints that will enable the ducting to expand or contract to accommodate such dimensional variations as may be caused by extremes of temperature variation. Otherwise, the ducting might be subject to leakage or failure entirely.
One example of an environment in which such ducting is necessary is that of a stationary gas turbine that is used for electrical power generation. Expansion joints for ducting for gas turbines must be able to accommodate relative axial movements of the duct ends on opposite side of the joint, as well as relative vertical and/or horizontal movements of the duct ends.
A joint for use with ducting such as used in association with such moving air or other gaseous flow sources is typically formed by creating a gap in the ducting (which ducting typically may be round or rectangular in cross-section). Inner and outer liner duct structures are then affixed to the opposing duct ends. The inner and outer liner duct structures are in overlapping telescopic relation to one another, with the inner liner duct structures on the upstream side of the joint. In this way, the force of the gas flow, during ordinary operating conditions, has less of a tendency to drive the gases between the overlapping portions of the inner and outer liner duct structures. A relatively close fit between the overlapping portions is provided, so that the impact of high-magnitude pulsations or turbulent flow in the air or other gaseous fluid flow, on the remaining surrounding expansion joint structure, is reduced.
To the outside of the liner duct structure, a plurality of insulating pillows is arranged, to provide protection to the outer components of the expansion joint from flow pulsations and the heat of the gas flow, and generally to reduce heat loss through the joint. Typically, several layers of insulating pillows are used.
Finally, a high-temperature flexible belt is provided to create the flexible outer skin of the expansion joint, connecting the portions of the duct on opposite sides of the gap. It is this high-temperature flexible belt that provides the actual sealing of the joint that prevents escape of the air or other gaseous flow, and in addition, prevents the loss of pressure along the ducting.
Typically, the flexible belt is fabricated from polytetrafluoroethylene (PTFE, typically sold under the trademark Teflon(copyright)), that may or may not have an additional layer of fiberglass, for added strength. However, the belt is usually simply applied as a substantially flat belt that encircles the joint, and is affixed to the duct ends, for example, by a series of bolts or studs (which may or may not actually pass through the upstream and downstream edges of the belt), to which a supporting ring (or more likely, series of arcuate ring segments) is attached, that clamps the ends of the belt to the upstream and downstream duct ends.
This belt, though relatively stiff, is not absolutely rigid and has no preformed profile. The belt is typically constructed to have a certain amount of xe2x80x9cslackxe2x80x9d in the longitudinal direction, even when the expansion joint is at its design maximum extension. However, under certain flow and pressure conditions, and also depending upon the state of relative compression of the joint, a portion of the belt may collapse or be sucked into the gap, toward the centerline of the ducting, causing the belt to invert and fold over onto itself. Because the belt is relatively stiff and not significantly stretchable under normal operating conditions, the circumference of the belt remains essentially constant, so while one portion of the belt is sucked in, another portion bulges out (as shown in FIG. 1). Such an effect causes creases and folds in the belt, which, in turn, can create hot spots, accelerated degradation, and potential premature failure of the belt.
Accordingly, it would be desirable to provide a flexible sealing belt for ducting expansion joints that resists collapse and buckling, when the expansion joint is in an unloaded or compressive physical orientation, or when a negative pressure differential exists, which would tend to draw the flexible sealing belt into the gap between the opposing duct ends.
This and other desirable characteristics of the invention will become apparent, in view of the present specification, including claims, and drawings.
The present invention is directed, in part, to a process for molding a sealing belt section for an expansion joint comprising the steps of:
providing a first mold half, having a substantially concave inside region having a desired shape;
providing a second mold half, having a substantially convex outside region having a desired shape,
the second mold half being operably configured to be substantially nestingly received by the first mold half;
placing a sheet of flexible sealing belt material between the first and second mold halves;
applying pressure to the mold halves, to force the sheet of flexible sealing belt material into the substantially concave inside region of the first mold half;
applying heat to the mold halves to, in turn, apply heat to the sheet of flexible sealing belt material;
holding the first and second mold halves together, once the first and second mold halves have been brought together to a desired proximity.
In a preferred embodiment of the invention, the step of placing a sheet of flexible sealing belt material further comprises the step of:
forming the sheet of flexible sealing belt material from a web of woven fiberglass material, and coating the web with PTFE.
The step of applying heat to the mold halves, preferably further comprises the step of:
placing heating elements against outwardly facing surfaces of the first and second mold halves.
The step of applying heat to the mold halves, preferably comprises the step of heating the mold halves to a temperature in the range of about 400xc2x0-680xc2x0 F.
In a preferred embodiment of the invention, the method may further comprise the step of:
applying insulation to the mold halves, to prevent loss of heat from the mold halves to ambient atmosphere.
The step of applying pressure to the mold halves preferably further comprises the steps of:
placing the first mold half on a surface;
placing the sheet of flexible sealing belt material atop the first mold half;
clamping the sheet of flexible sealing belt material to the first mold half;
placing the second mold half atop the sheet of flexible sealing belt material;
placing a weight receiving frame on the second mold half; and
placing one or more weights on the weight receiving frame.
In a preferred embodiment of the invention, the method may further comprise the step of:
rocking the second mold half, to prompt movement of the sheet of flexible sealing belt material into the first mold half, as the sheet of flexible sealing belt material becomes heated.
The step of holding the first and second mold halves together preferably comprises the step of:
clamping the mold halves together.
In a preferred embodiment of the invention, the method may further comprise the steps of:
continuing to apply heat to the first and second mold halves, while the first and second mold halves are being held together.
In a preferred embodiment of the invention, the method may further comprise the steps of:
ending application of heat after an amount of time in the range of about 1-2 hours;
permitting the mold halves to cool;
removing the molded sealing belt section, once the mold halves have cooled to a temperature in the range of about 200xc2x0-300xc2x0 F.
The present invention is also directed, in part, to a sealing belt section for an expansion joint comprising a flexible sealing belt member, operably configured for peripherally surrounding at least a portion of the periphery of an expansion joint. The flexible sealing belt member is preferably fabricated from a sheet of at least one layer of flexible sealing belt material.
The flexible sealing belt member is preferably further provided with a pre-formed biased outwardly extending cross-sectional configuration, so that upon installation of the flexible sealing belt member into an expansion joint, connecting opposing duct ends, the flexible sealing belt member will be prompted to remain in an biased outwardly extending cross-sectional configuration, when the opposing duct ends of the expansion joint are brought toward one another.